OFFSET
0,2
COMMENTS
Consider a classification of the positive numbers with classes {1}, A000040 (primes), A156759 (n>=2) (preprimes, or preprimes of the first kind), A247393 (preprimes of the second kind), A247394 (preprimes of the third kind), etc.
Then a(0)=1, a(1)=2; for n>=3, a(n) is the smallest number which is a preprime of the (n-1)st kind.
LINKS
Indranil Ghosh, Table of n, a(n) for n = 0..1000
Vladimir Shevelev, A classification of the positive integers over primes
FORMULA
For n>=3, a(n) = (prime(n-1))^2 + 1.
MATHEMATICA
Table[If[n>2, Prime[n - 1]^2 + 1, 2^n], {n, 0, 43}] (* Indranil Ghosh, Mar 08 2017 *)
PROG
(PARI) a(n)=if(n>2, (prime(n-1))^2 + 1, 2^n) \\ Charles R Greathouse IV, Sep 17 2014
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Vladimir Shevelev, Sep 16 2014
EXTENSIONS
More terms from Peter J. C. Moses, Sep 16 2014
STATUS
approved